The US suffered a tremendous loss Saturday in Afganistan when the downing of a Chinook helicopter took the lives of 31 special forces troops. As Danger Room reports, it may have been an entirely new Taliban weapon that did it:

The passengers and crew of the twin-rotor CH-47 Chinook helicopter probably never saw the rocket hurtling towards them. The explosion and fiery crash in Wardak province in eastern Afghanistan early on Saturday morning killed all 38 people aboard the lumbering chopper.

For U.S. forces, it was the bloodiest single incident of the 10-year-old Afghanistan war - and possibly a sign of the insurgency's continued ability to introduce new weaponry. The attack is also a chilling reminder of the vulnerability of the U.S.-lead coalition's indispensable helicopters. "Shock and disbelief," is how one official characterized the reaction inside the military.

Details of the shootdown are slowly emerging. "There will be multiple investigations," a Special Operations Command official said.

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Sometime late Friday, it appears, a team of U.S. Army Rangers got pinned down by insurgent fighters during a patrol in Wardak, a province just south of Kabul that, along with neighboring Logar province, is a major staging area for the Taliban and other insurgent groups.

The Rangers called in their "Immediate Reaction Force," a helicopter-borne mobile reserve that orbits nearby during risky patrols. That day, IRF duty had fallen to the Navy SEALs and their attachments, part of the 10,000-strong Afghanistan-based Joint Special Operations Command task force that, in addition to killing Osama bin Laden in May, also conducts as many as 70 raids per day in Afghanistan and Pakistan. In 2,800 raids between April and July, JSOC captured around 2,900 insurgents and killed more than 800, military sources said. That's twice as many raids compared to the same period a year ago.

Normally, JSOC commandos ride in tricked-out helicopters - including stealth models - belonging to the Army's 160th Special Operations Aviation Regiment. But this weekend the SEALs hitched a ride in what was apparently a run-of-the-mill Army National Guard chopper.

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With the SEALs' help, the Rangers fought back against their ambushers. Eight insurgents died in the fighting, according to a Taliban spokesman. Believing the battle over, around 3:00 in the morning, local time, the SEALs and their allies climbed back into their CH-47 for the ride home. That's when all Hell broke loose.

"The Taliban knew which route the helicopter would take," one unnamed Afghan official tells AFP. "That's the only route, so they took position[s] on the either side of the valley on mountains and as the helicopter approached, they attacked it with rockets and other modern weapons."

"It was a trap that was set by a Taliban commander," the official added.

The aircraft fell to the ground in flames.

Chopper-Killer

The cause of the CH-47 crash is still under investigation. "The helicopter was reportedly fired on by an insurgent rocket-propelled grenade," according to a coalition press release. Which weapon - or weapons - were actually responsible for the copter coming down is not yet known. Several publications claim an insurgent Rocket-Propelled Grenade struck the helicopter.

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One Army insider who spoke to Danger Room went a step further, saying the rocket may have been a special improvised model. A chopper-killer, if you will.

IRAMs have killed several U.S. troops in Iraq over the years; in June, the weapons killed six Americans. but haven't factored heavily in the Afghanistan fighting. The weapon's appearance in Wardak, if confirmed, could be proof of Afghan insurgents' continued ability to adapt and innovate despite mounting losses.

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Improvised rockets are notoriously inaccurate. But with bigger warheads than shoulder-fired RPGs, IRAMs are potentially much more destructive when they do hit.

Not that it takes much to bring down a helicopter. Complex, slow and low-flying, choppers have always been vulnerable to attack from the ground. The coalition has lost hundreds of helicopters in Afghanistan over the last decade.

That the SEALs in Wardak were flying in a National Guard CH-47 probably didn't make any difference. "Nothing about the aircraft would really make it more susceptible to ground fire than, say, a regular Army aircraft or a Special Ops bird," the Army insider said.

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Though enhanced Special Operations helicopters boast better navigation systems and, in some cases, even stealthy outer shells, they're no more able to absorb an unguided rocket than any other copter. And for helicopters, there's no effective countermeasure for unguided attacks besides aggressive flying, which isn't really possible while the aircraft is close to the ground and full of troops.

That places huge demands on the aircraft and their operators. This is one subject of my forthcoming book From A to B. "My biggest headache is vertical lift," Army Lt. Col. Thomas Gukeisen, in 2009 the commander of a combined U.S. and Czech force in Logar, told me for the book. "Vertical lift" is Army jargon for choppers.

The IRAM's possible appearance in Afghanistan could make helicopters more vulnerable than they already are. At the same time, nothing short of a Herculean road-building effort - or a sudden, massive troop reduction - can quickly reduce the huge demand for rotorcraft.

In comments to reporters, NATO spokesman Brigadier General Carsten Jacobsen appeared to swat away the possibility that an IRAM was employed in the Wardak attack.

Whether or not an IRAM counts as something "new" to Jacobsen is unclear; we're following up to find out. But even subtracting the IRAM, the result of that awful arithmetic is more crashed choppers and more dead coalition troops, on a regular basis until the war ends. Saturday's shootdown was an unusually bloody copter tragedy, but it's hardly the first for the Afghan war. And it won't be the last.